U.S. patent application number 11/002317 was filed with the patent office on 2005-08-25 for apparatus and method for estimating initial frequency offset in an asynchronous mobile communication system.
This patent application is currently assigned to Samsung Electronics Co., Ltd.. Invention is credited to Hur, Seong-Ho.
Application Number | 20050186924 11/002317 |
Document ID | / |
Family ID | 34698970 |
Filed Date | 2005-08-25 |
United States Patent
Application |
20050186924 |
Kind Code |
A1 |
Hur, Seong-Ho |
August 25, 2005 |
Apparatus and method for estimating initial frequency offset in an
asynchronous mobile communication system
Abstract
An apparatus and method for estimating an initial frequency
offset in a mobile communication system in which a user equipment
(UE) performs initial cell search in order to identify a Node B
with which the UE can exchange data is provided. The apparatus
comprises a memory for storing a plurality of pulse duration
modulation (PDM) hypotheses and storing the PDM hypotheses therein;
a step#1 cell searcher for performing a step#1 cell search on each
of the PDM hypotheses through a primary synchronization channel and
outputting the cell search result to the memory; and an initial
frequency offset estimator for determining an initial frequency
offset estimation value from the cell search results for the PDM
hypotheses.
Inventors: |
Hur, Seong-Ho; (Suwon-Si,
KR) |
Correspondence
Address: |
ROYLANCE, ABRAMS, BERDO & GOODMAN, L.L.P.
1300 19TH STREET, N.W.
SUITE 600
WASHINGTON,
DC
20036
US
|
Assignee: |
Samsung Electronics Co.,
Ltd.
|
Family ID: |
34698970 |
Appl. No.: |
11/002317 |
Filed: |
December 3, 2004 |
Current U.S.
Class: |
455/161.1 ;
375/E1.005; 455/423; 455/62 |
Current CPC
Class: |
H04B 1/7087 20130101;
H04L 2027/0034 20130101; H04B 1/7083 20130101 |
Class at
Publication: |
455/161.1 ;
455/423; 455/062 |
International
Class: |
H04B 007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 11, 2004 |
KR |
2004-8988 |
Claims
What is claimed is:
1. A method for setting an initial frequency by a user equipment
(UE) in a mobile communication system in which the UE performs an
initial cell search in order to identify a Node B with which the UE
can exchange data, the method comprising the steps of: performing a
step#1 cell search on predetermined frequency hypothesis hypotheses
through a primary synchronization channel; storing the cell search
result; determining an initial frequency offset estimation value
from the cell search results for the predetermined frequency
hypothesis; and estimating a frequency offset using the determined
initial frequency offset estimation value.
2. The method of claim 1, wherein the predetermined frequency
hypothesis is pulse duration modulation (PDM) value and the PDM
hypotheses are located at regular intervals in a loop bandwidth of
an automatic frequency controller.
3. The method of claim 1, wherein the step#1 cell search result on
each of the PDM hypotheses comprises a maximum energy value.
4. The method of claim 1, wherein the step of determining an
initial frequency offset estimation value from the cell search
results for the PDM hypotheses comprises the step of determining a
PDM hypothesis having a maximum energy value among the cell search
results.
5. The method of claim 1, wherein the step of determining an
initial frequency offset estimation value from the cell search
results for the PDM hypotheses comprises the step of determining a
value obtained by averaging a predetermined number of PDM
hypotheses having a higher energy value among the cell search
results.
6. The method of claim 1, further comprising the step of estimating
a frequency offset by an automatic frequency controller after
estimating a frequency offset based on the determined initial
frequency offset estimation value.
7. An apparatus for setting an initial frequency in a mobile
communication system in which a user equipment (UE) performs an
initial cell search in order to identify a Node B with which the UE
can exchange data, the apparatus comprising: a memory for storing a
plurality of predetermined frequency hypothesis; a step#1 cell
searcher for performing a step#1 cell search on each of the
predetermined frequency hypotheses through a primary
synchronization channel and outputting the cell search result to
the memory; and an initial frequency offset estimator for
determining an initial frequency offset estimation value from the
cell search results for the PDM hypotheses.
8. The apparatus of claim 7, wherein the predetermined frequency
hypothesis is pulse duration modulation (PDM) value and the PDM
hypotheses are located at regular intervals in a loop bandwidth of
an automatic frequency controller.
9. The apparatus of claim 7, wherein the step#1 cell search result
on each of the PDM hypotheses comprises a maximum energy value.
10. The apparatus of claim 7, wherein the initial frequency offset
estimator determines a PDM hypothesis having a maximum energy value
among the cell search results.
11. The apparatus of claim 7, wherein the initial frequency offset
estimator determines a value obtained by averaging a predetermined
number of PDM hypotheses having a higher energy value among the
cell search results.
12. The apparatus of claim 7, further comprising an automatic
frequency controller for estimating a frequency offset for a
received signal based on a common pilot channel.
13. The apparatus of claim 12, wherein the automatic frequency
controller includes a frequency difference detector for detecting a
frequency difference between a received signal and a frequency
offset-estimated signal.
14. The apparatus of claim 13, further comprising a controller for
selecting one of the initial frequency offset estimation value
determined by the initial frequency offset estimator and an output
value of the frequency difference detector.
15. The apparatus of claim 14, wherein the controller selects the
output value of the frequency difference detector and performs
frequency offset estimation on the selected output value by the
automatic frequency controller after an initial frequency offset is
estimated based on the initial frequency offset estimation value
determined by the initial frequency offset estimator.
Description
PRIORITY
[0001] This application claims priority under 35 U.S.C. .sctn.
119(a) to an application entitled "Apparatus and Method for
Estimating Initial Frequency Offset in an Asynchronous Mobile
Communication System" filed in the Korean Intellectual Property
Office on Feb. 11, 2004 and assigned Ser. No. 2004-8988, the entire
contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to an asynchronous
mobile communication system. In particular, the present invention
relates to an apparatus and method for estimating an initial
frequency offset.
[0004] 2. Description of the Related Art
[0005] Mobile communication systems can typically be classified
into synchronous systems and asynchronous systems. The synchronous
systems have mainly been adopted in the United States, while the
asynchronous systems have been mainly adopted in Europe.
[0006] With the recent rapid growth of the mobile communication
industry, future mobile communication systems capable of supporting
not only voice service but also data and image services are
attracting public attention, and standardization work on the future
mobile communication systems is being conducted. However, the
United States and Europe which are adopting different mobile
communication systems are each independently carrying out separate
standardizations. The European future mobile communication system
is one of the new standards and is known as a Universal Mobile
Telecommunication System (UMTS).
[0007] Typically, in order to search for a Node B (or base station
transceiver subsystem (BTS)), a user equipment (UE; or mobile
station) comprising a mobile communication system requires the
performance of frequency offset estimation and compensation for a
carrier frequency. Frequency offset estimation and compensation
greatly affect a Node B search time and call quality. The term
"frequency offset" refers to a frequency variation occurring when a
carrier frequency received from a Node B varies according to
various factors (e.g., Node B signal distortion or a Doppler
frequency) due to a channel environment, and a UE performs
frequency offset estimation and compensation in order to match a
transmission/reception frequency of the UE with a
transmission/reception frequency of a Node B by removing the
frequency offset.
[0008] For example, if a carrier frequency is 2.14 GHz, a carrier
frequency F.sub.r that a UE receives becomes 2.14
GHz+.DELTA.f.sub.r as a specific frequency offset .DELTA.f.sub.r is
added to the carrier frequency 2.14 GHz generated in a transmission
side. Therefore, the UE can normally restore a received signal by
estimating and compensating for the frequency offset
.DELTA.f.sub.r. Generally, the frequency offset .DELTA.f.sub.r is a
value in which a frequency distortion
.DELTA.f.sub.drift.sub..sub.--.sub.in.sub..sub.--.sub.BTS in a Node
B and a Doppler frequency .DELTA.f.sub.D are reflected.
[0009] Frequency offset estimation by a UE is performed through an
automatic frequency controller (hereinafter referred to as "AFC").
An operating principle of the AFC is to correct a frequency of a
received signal by comparing a received signal's frequency F.sub.r
with a carrier frequency in which an estimated specific frequency
offset .DELTA.f.sub.r is reflected and continuously compensating
for a difference. A detailed implementation method will be
described below.
[0010] The AFC has two limitations: it cannot trace all frequency
offsets because of the limited loop bandwidth, and it must know a
cell scrambling code and its timing. In other words, it can operate
after completion of a cell search. However, in an asynchronous
mobile communication system, because it is not possible to know a
cell scrambling code before acquiring a cell, the AFC cannot be
used. Therefore, estimating a frequency offset before performing
cell search is required, and a temperature-based frequency
estimation method used in a UE supporting a synchronous mobile
communication system can be used as a method for estimating an
initial frequency offset. With reference to FIG. 1, a description
will now be made of a temperature-based initial frequency
estimation method according to the prior art.
[0011] FIG. 1 is a block diagram illustrating a frequency offset
estimation apparatus in an asynchronous mobile communication system
according to the prior art. Referring to FIG. 1, the frequency
offset estimation apparatus can comprise a first multiplier 101, a
low pass filter (LPF) 103, an analog-to-digital converter (ADC)
105, a second multiplier 107, a frequency difference detector (FDD)
109, a temperature sensor 111, a memory 113, a controller 115, an
accumulator 117, a pulse duration modulator (PDM) 119, a voltage
controlled oscillator (VCO) 121 and a frequency multiplier 123.
[0012] The FDD 109 divides an output signal of the second
multiplier 107 into I-channel symbols and Q-channel symbols, and
detects a frequency difference by performing a specific operation
(e.g., I(n)Q(n-1)-I(n-1)Q(n)) on current symbols and previous
symbols. In general, a frequency difference detected through the
above operation is output after detection results for 4 symbols are
reflected (i.e., accumulated).
[0013] The controller 115 selects an output value from the FDD 109
and a value read from the memory 113, and outputs the selected
value to the accumulator 117. As stated, in an initial state,
because the AFC does not normally operate, the controller 115 reads
as an initial frequency offset a value stored in the memory 113
instead of the output value of the FDD 109 and outputs the read
value to the accumulator 117.
[0014] More specifically, the temperature-based frequency
estimation method according to the prior art estimates an initial
frequency offset by measuring a temperature by means of the
temperature sensor 111 and by reading a frequency offset
corresponding to the measured temperature from a table stored in
the memory 113, in which relationships between temperatures and
frequency offsets are stored. Therefore, the controller 115
receives an ambient temperature value of the voltage controlled
oscillator 121 from the temperature sensor 111, reads a frequency
offset corresponding to the measured temperature based on the
relationships between temperatures and frequency offsets,
previously stored in the memory 113, and outputs the read frequency
offset to the accumulator 117. The accumulator 117 accumulates a
currently received value to add to a previously stored value, and
outputs the accumulated value to the PDM 119. The PDM 119 generates
a pulse corresponding to an initial frequency offset generated
according to the temperature, and outputs the generated pulse to
the voltage controlled oscillator 121.
[0015] The voltage controlled oscillator 121 generates a specific
oscillation frequency according to the pulse value output from the
PDM 119. The oscillation frequency output from the voltage
controlled oscillator 121 is subject to frequency multiplication in
the frequency multiplier 123, generating a carrier frequency (i.e.,
radio frequency). An output of the frequency multiplier 123 is
multiplied by a received signal in the first multiplier 101.
[0016] If an initial frequency offset is estimated in this manner
by the initial frequency offset generated according to a
temperature, estimation of a next frequency offset is performed by
an AFC through the above-stated AFC operation. Meanwhile, in the
initial frequency offset estimation process, because the AFC does
not normally operate as stated above, the controller 115 selected
the value read from the memory 113. However, when the AFC normally
operates by the initial frequency offset estimation, the controller
115 selects the output value of the FDD 109. That is, as a loop of
the AFC continuously performs a normal operation, although a
received frequency is changed, it is possible to trace the changed
frequency offset.
[0017] For the voltage controlled oscillator 121, an oven
controlled temperature compensated crystal oscillator (OCTCXO) or a
voltage controlled temperature compensated crystal oscillator
(VCTCXO) is used as a reference frequency generator.
[0018] Meanwhile, in the asynchronous mobile communication system,
the temperature-based frequency estimation method used in the
synchronous mobile communication system can be used as a method for
estimating an initial frequency offset before completion of a cell
search. As stated, the temperature-based frequency estimation
method estimates an initial frequency offset by measuring an
ambient temperature of a place where the VCTCXO operates and by
reading a frequency offset corresponding to the measured
temperature from a table in which a relationship between
temperatures and frequency offsets are stored.
[0019] However, because such a method estimates an initial
frequency offset based on a table in which relationships between
temperatures and frequency offsets are stored, an extra memory for
storing the relationships is required, a unique table must be set
up for each VCTCXO used in each UE, and the table must be changed
after a run time of the VCTCXO. Actually, the VCTCXOs, though they
are the same model made by the same company, show considerably
different characteristics, and the characteristics vary undesirably
over time.
[0020] In addition, although an initial frequency offset is
estimated based on a value stored in the table in order to
compensate for an influence of temperature, an initial frequency
offset between a frequency transmitted by a Node B and a frequency
generated by a UE to receive a signal transmitted by the Node B can
show a considerably large value. Actually, in a 3.sup.rd Generation
Partnership Project (3GPP) asynchronous system, when a Doppler
frequency is taken into consideration, an initial carrier frequency
offset is about 7.5 KHz and a system clock frequency offset is
about 100 Hz.
[0021] When an initial frequency offset is considerably large, an
AFC fails in frequency offset compensation and a long time is
required until a frequency offset is compensated. That is, a very
long time is required until a frequency offset is corrected because
after a step#3 cell search is performed with an initially estimated
frequency offset, if the cell search failed, an accurate frequency
offset is detected by reading another frequency offset from a table
in which relationships between temperatures and frequency offsets
are stored and repeating the step#3 cell search for the read
frequency offset. This causes call failure or a deterioration in
call quality.
SUMMARY OF THE INVENTION
[0022] It is, therefore, an object of the present invention to
provide an apparatus and method for efficiently and reliably
estimating an initial frequency offset by using a step#1 cell
search in an asynchronous mobile communication system.
[0023] To achieve the above and other objects, there is provided a
method for estimating an initial frequency offset by a user
equipment (UE) in a mobile communication system in which the UE
performs an initial cell search in order to identify a Node B with
which the UE can exchange data. The method comprising the steps of
performing step#1 cell search on each of pulse duration modulation
(PDM) hypotheses through a primary synchronization channel; storing
the cell search result; determining an initial frequency offset
estimation value from the cell search results for the PDM
hypotheses; and estimating a frequency offset using the determined
initial frequency offset estimation value.
[0024] To achieve the above and other objects, there is provided an
apparatus for estimating an initial frequency offset in a mobile
communication system in which a user equipment (UE) performs an
initial cell search in order to identify a Node B with which the UE
can exchange data. The apparatus comprising a memory for storing a
plurality of pulse duration modulation (PDM) hypotheses and storing
the PDM hypotheses therein; a step#1 cell searcher for performing
step#1 cell search on each of the PDM hypotheses through a primary
synchronization channel and outputting the cell search result to
the memory; and an initial frequency offset estimator for
determining an initial frequency offset estimation value from the
cell search results for the PDM hypotheses.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The above and other objects, features and advantages of the
present invention will become more apparent from the following
detailed description when taken in conjunction with the
accompanying drawings in which:
[0026] FIG. 1 is a block diagram illustrating a frequency offset
estimation apparatus in an asynchronous mobile communication system
according to the prior art;
[0027] FIG. 2 is a block diagram illustrating an initial frequency
offset estimation apparatus using step#1 cell search according to
an embodiment of the present invention;
[0028] FIG. 3 is a block diagram illustrating a structure of a
step#1 cell searcher used in an asynchronous mobile communication
system according to an embodiment of the present invention;
[0029] FIG. 4 is a diagram illustrating a method for storing pulse
duration modulation (PDM) hypotheses according to an embodiment of
the present invention; and
[0030] FIG. 5 is a flowchart illustrating an initial frequency
offset estimation method using step#1 cell search according to an
embodiment of the present invention.
[0031] In the drawings, it should be understood that like reference
numbers refer to like features and structures.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0032] Several embodiments of the present invention will now be
described in detail with reference to the accompanying drawings. In
the following description, a detailed description of known
functions and configurations incorporated herein has been omitted
for conciseness.
[0033] The present invention provides a more effective, and more
reliable initial frequency offset estimation method using a primary
synchronization channel (P-SCH) used for cell searching in an
asynchronous mobile communication system. Therefore, because a
current channel condition can be correctly reflected, the new
initial frequency offset estimation method is more reliable than
the conventional temperature-based initial frequency offset
estimation method. In addition, a separate table for storing
temperature-frequency offset relations is not required, and
reliable initial frequency offset estimation is available
irrespective of characteristic variations due to instability of the
VCTCXO.
[0034] A UE supporting an asynchronous mobile communication system
performs a 3-step cell search including a first step (herein
referred to as step#1) cell search, a second step (herein referred
to as step#2) cell search and a third step (herein referred to as
step#3) cell search for cell acquisition (i.e., Node B
identification). The step#1 cell search acquires slot
synchronization using P-SCH, the step#2 cell search acquires Node B
group code-related information and frame synchronization by a
secondary synchronization channel (S-SCH), and the step#3 cell
search acquires a Node B code using a common pilot channel
(CPICH).
[0035] Because embodiments of the present invention estimate an
initial frequency offset using a step#1 cell searcher that performs
the step#1 cell search using P-SCH, the estimation can be achieved
by combining existing hardware without additional hardware.
[0036] With reference to FIG. 2, a detailed description will now be
made of a frequency offset estimation method for estimating an
initial frequency offset using a step#1 cell search according to an
embodiment of the present invention.
[0037] FIG. 2 is a block diagram illustrating an initial frequency
offset estimation apparatus using a step#1 cell search according to
an embodiment of the present invention. Referring to FIG. 2, the
frequency offset estimation apparatus according to an embodiment of
the present invention can comprise a first multiplier 201, a low
pass filter (LPF) 203, an analog-to-digital converter (ADC) 205, a
second multiplier 207, a step#1 cell searcher 209, a memory 211, an
initial frequency offset estimator 213, a third multiplier 215, a
frequency difference detector (FDD) 217, a controller 219, an
accumulator 221, a pulse duration modulator (PDM) 223, a voltage
controlled oscillator (VCO) 225, and a frequency multiplier
227.
[0038] While the conventional temperature-based initial frequency
offset estimation apparatus estimates an initial frequency offset
using a table representing relationships between temperatures and
frequency offsets, a new initial frequency offset estimation
apparatus according to an embodiment of the present invention sets
up a plurality of PDM hypotheses and estimates as an initial
frequency offset a frequency offset for PDM in which a maximum peak
energy value is detected through a step#1 cell search for each PDM
hypothesis. That is, the new initial frequency offset estimation
apparatus divides PDM values providing a control voltage for a
temperature compensated crystal oscillator (TCXO) provided to
generate an oscillation frequency at predetermined intervals and
estimates an initial frequency offset from a result of the step#1
cell search on each PDM value.
[0039] The PDM hypothesis values are stored in the memory 211, and
the step#1 cell search is performed on each PDM hypothesis. A
result of the step#1 cell search on each PDM hypothesis is stored
in the memory 211 or a separate memory. However, for convenience,
it will be assumed herein that the result of the step#1 cell search
on each PDM hypothesis is stored in the memory 211. The initial
frequency offset estimator 213 estimates an initial frequency
offset by determining an optimal PDM value from the step#1 cell
search results on all of the PDM hypotheses stored in the memory
211. A method for determining an optimal PDM value from the step#1
cell search results can be implemented in several ways, and a
detailed description thereof will be provided below.
[0040] First, a method for performing the step#1 cell search on
each PDM hypothesis will be described in more detail. As stated,
the PDM hypotheses are stored in the memory 211, and the step#1
cell search is sequentially performed on the PDM hypotheses. When
the controller 219 reads a data value for one of the PDM hypothesis
stored in the memory 211, it applies the read data value for the
PDM hypothesis to the accumulator 221.
[0041] When the controller 219 inputs an output of the FDD 217 to
the accumulator 221, the accumulator 221 adds the current input
value to a previously stored value and provides the output to the
PDM 223. However, when the controller 219 inputs a PDM hypothesis
read from the memory 211 to the accumulator 221, the accumulator
221 bypasses the read PDM hypothesis and provides an output to the
PDM 223. The PDM 223 generates a pulse corresponding to a data
value for the read PDM hypothesis and outputs the generated pulse
to the voltage controlled oscillator 225.
[0042] The voltage controlled oscillator 225, a device for
generating a specific oscillation frequency according to an input
voltage value, generates a specific oscillation frequency according
to a pulse value output from the PDM 223. An oscillation frequency
output from the voltage controlled oscillator 225 is subject to
frequency multiplication in the frequency multiplier 227,
outputting a carrier frequency (i.e., radio frequency). An output
of the frequency multiplier 227 is multiplied by a received signal
in the first multiplier 201.
[0043] A reception signal F.sub.r received through an antenna is
multiplied by a signal generated based on the PDM hypothesis by the
first multiplier 201, and an output signal of the first multiplier
201 is multiplied by a P-SCH signal in the second multiplier 207
after passing through the LPF 203 and the ADC 205. An output signal
of the second multiplier 207 is input to the step#1 cell searcher
209, and the step#1 cell searcher 209 performs a step#1 cell search
procedure. An operating procedure of the step#1 cell searcher 209
is defined in a 3GPP standard specification which is incorporated
herein in its entirety by reference, and is generally made in the
form of a pruned efficient Golay correlator (PEGC).
[0044] When the step#1 cell searcher 209 completes the step#1 cell
search on one of the PDM hypothesis, it stores the cell search
result in the memory 211 as a peak energy value. If the step#1 cell
search on all PDM hypotheses is completed in this manner, peak
energy values for the PDM hypotheses are stored in the memory 211.
The initial frequency offset estimator 213 compares the peak energy
values for the PDM hypotheses stored in the memory 211 and
determines an initial frequency offset estimation value based on a
PDM hypothesis having a maximum peak energy value or an average
value for a predetermined number of PDM hypotheses having a higher
peak energy value. The method for determining an initial frequency
offset estimation value from the step#1 cell search results for the
PDM hypotheses can be modified in several ways, and a detailed
description thereof will be provided below.
[0045] The controller 219 receives an initial frequency offset
estimation value which was determined by the initial frequency
offset estimator 213 and outputs the received initial frequency
offset estimation value to the accumulator 221. The accumulator 221
bypasses a PDM hypothesis having the maximum peak energy value read
from the memory 211 to the PDM 223. The PDM 223 generates a pulse
corresponding to an initial frequency offset determined according
to an embodiment of the present invention, and outputs the
generated pulse to the voltage controlled oscillator 225.
[0046] The voltage controlled oscillator 225, a device for
generating a specific oscillation frequency according to an input
voltage value, generates a specific oscillation frequency according
to a pulse value output from the PDM 223. An oscillation frequency
output form the voltage controlled oscillator 225 is subject to
frequency multiplication in the frequency multiplier 227,
outputting a carrier frequency (i.e., radio frequency). An output
of the frequency multiplier 227 is multiplied by a received signal
in the first multiplier 201.
[0047] If an initial frequency offset is estimated in this way
based on an initial frequency offset generated according to an
embodiment of the present invention, next frequency offset
estimation is subject to automatic frequency control by the
above-described AFC's operation. That is, in order to estimate an
accurate frequency offset from an output value determined by an
operation between a signal received at the first multiplier 201 and
an estimated signal, an output signal of the first multiplier 201
passes through the LPF 203, the ADC 205, the third multiplier 215
and the FDD 217. Meanwhile, in the initial frequency offset
estimation process, because the AFC does not normally operate as
stated above, the controller 219 selected the value read from the
memory 213. However, from a time when the AFC normally operates by
the initial frequency offset estimation, the controller 219 selects
the output value of the FDD 217. That is, as a loop of the AFC
continuously performs a normal operation, although a received
frequency is changed, it is possible to trace the changed frequency
offset.
[0048] A structure of the step#1 cell searcher 209 used to
implement the present invention will now be described in more
detail with reference to FIG. 3. The detailed structure of the
step#1 cell searcher is shown as an example. However, the
embodiment of the present invention is not limited to the example
shown. Therefore, it is obvious that each of the blocks illustrated
in FIG. 3 can be modified for efficient step#1 cell search.
Embodiments of the present invention are characterized by deterring
an initial frequency offset estimation value using an output value
of the step#1 cell searcher.
[0049] FIG. 3 is a block diagram illustrating a structure of a
step#1 cell searcher used in an asynchronous mobile communication
system according to an embodiment of the present invention.
Referring to FIG. 3, the step#1 cell searcher 209 of FIG. 2 detects
a correlation value between a signal received at a UE and a P-SCH
signal generated in the UE at all hypothesis points arranged at
specified intervals, or intervals of 1/2 chip within one slot. The
step#1 cell searcher 209 detects from the detected correlation
values a plurality of slot timings having a correlation value that
is a peak value and is higher than a predetermined threshold.
[0050] When a P-SCH signal comprised of an I-channel signal and a
Q-channel signal is received at the step#1 cell searcher 209, the
received P-SCH signal is applied to a decimator 311. Here, 2S
(S=0,1,2, . . . ) P-SCH signals are input to the decimator 311 per
chip. The decimator 311 selects 2M P-SCH signals (where M<S),
for which 2S P-SCH signals are received per chip, and outputs the
selected signals in parallel. The P-SCH signals output in parallel
from the decimator 311 are input to correlators 313, 315, 317 and
319, respectively. Here, the required number of the correlators is
equal to the number of I-channel and Q-channel components for the
2M P-SCH signals output from the decimator 311.
[0051] Among the correlators, the correlators 313 and 315 detect
correlation values with a P-SCH code generated by the step#1 cell
searcher 209 for on-time I-channel and on-time Q-channel signal
components of the P-SCH signal, while the correlators 317 and 319
detect correlation values with the P-SCH code for late-time
I-channel and late-time Q-channel signal components for the P-SCH
signal. Here, the correlators 313, 315, 317 and 319, being pruned
efficient Golay correlators (PEGCs), calculate correlation values
between the received I-channel or Q-channel signals and the P-SCH
code, and output the calculated correlation values per chip. The
PEGC is a kind of a matched filter, and its filter depth is 256
chips because a symbol size of P-SCH is 256 chips. If the PEGC
receives 256-chip data in addition to 1-slot reception data, it
indicates timing having a maximum value and its energy.
[0052] Correlation values output from the correlators 313 and 315
that detect correlation values corresponding to on-time I-channel
and on-time Q-channel signal components of the P-SCH signal are
output to an energy calculator 321 that detects correlation energy
for the on-tine I-channel and on-time Q-channel signal components
of the P-SCH signal. Similarly, correlation values output from the
correlators 317 and 319 that detect correlation values
corresponding to late-time I-channel and late-time Q-channel signal
components of the P-SCH signal are output to an energy calculator
323 that detects correlation energy for the late-tine I-channel and
late-time Q-channel signal components of the P-SCH signal. The
energy calculator 321 squares the correlation values for the
on-time I-channel and on-time Q-channel signal components of the
P-SCH signal, output from the correlators 313 and 315, adds up the
squared correlation values, and outputs the added value to a
parallel-to-signal (P/S) converter 325. Similarly, the energy
calculator 323 squares the correlation values for the late-time
I-channel and late-time Q-channel signal components of the P-SCH
signal, output from the correlators 317 and 319, adds up the
squared correlation values, and outputs the added value to the P/S
converter 325.
[0053] The P/S converter 325 serial-converts the correlation
energies for the on-time I-channel and on-time Q-channel components
of the P-SCH signal and the late-time I-channel and late-time
Q-channel components of the P-SCH signal, output from the energy
calculators 321 and 323. Here, the P/S converter 325 receives the
correlation energies at intervals of Tc/2M, and sequentially
outputs the received correlation energies, i.e., 2M correlation
energies. The 2M correlation energies output from the P/S converter
325 are input to an accumulator 327. The accumulator 327 adds the
correlation energies output from the P/S converter 325 to the
accumulated correlation energies for corresponding hypothesis
points for a predetermined number of times. After completion of the
accumulation, the accumulator 327 outputs the total correlation
energy to a peak detector 329.
[0054] The peak detector 329 then detects K1 accumulated
correlation energies having a maximum energy value while being a
peak value from the accumulated correlation energies for
2560.times.2M hypothesis points, output from the accumulator 327. A
controller (not shown) compares the K1 accumulated correlation
energies detected by the peak detector 329 to a predetermined
threshold, and if there is any accumulated correlation energy
higher than the threshold, the controller determines that the
step#1 cell search is completed.
[0055] Therefore, the step#1 cell search is performed on each of a
plurality of predetermined PDM hypotheses, and a peak value for
each of the PDM hypotheses is stored as stated. In addition, an
initial frequency offset is estimated from the stored peak
values.
[0056] A method for setting up the PDM hypotheses will now be
described in detail with reference to FIG. 4. The method for
setting up the PDM hypotheses can also be implemented in several
ways, and it is preferable to set up the PDM hypotheses by dividing
the entire PDM interval at predetermined intervals.
[0057] For example, it is assumed herein that the PDM comprises N
bits and its value has a 2's complement. In this case, the PDM has
a value of -2.sup.N-1-2.sup.N-1-1. That is, if the PDM comprises 9
bits, it has a value of -256.about.255.
[0058] In order to set up the PDM hypotheses, a PDM space is first
calculated. The PDM space is set up so as to be narrower than or
equal to a bandwidth of AFC, i.e., a PDM range that can be
independently traced by AFC hardware, used for frequency offset
estimation after completion of cell search after a cell scrambling
code is acquired through experiment or simulation. Because the
initial frequency offset estimation value will be used in the AFC
hardware, it is preferable to set the PDM space to a value shorter
than the hardware trace range. Here, the set PDM space is referred
to as `S`.
[0059] Thereafter, the PDM hypotheses PH are calculated. The PDM
hypotheses mean values are read by the controller 219 of FIG. 2
from the memory 211 for an initial frequency offset estimation
test. PDM hypotheses to be used are calculated by a value S
previously determined within the PDM interval. Therefore, the total
number L of the PDM hypotheses becomes [2.sup.N/S]. Here, [.]
denotes a Gaussian sign and indicates a value below a decimal point
is discarded. For example, if it is assumed that a 9-bit PDM is
used and its PDM space is S=20, L=[512/20]=25. That is, 25 PDM
hypotheses exist.
[0060] Next, the step#1 cell search described in connection with
FIGS. 2 and 3 is performed on each of the calculated L PDM
hypotheses. As a result of the step#1 cell search, a maximum energy
and its corresponding PDM hypothesis are stored in the memory 211
for each of the PDM hypotheses. In this case, energies for all the
hypotheses can be stored, or only the energies whose levels belong
to the upper M PDM hypotheses can be stored for efficient
utilization of the memory. In this manner, the upper M PDM
hypotheses and their energies are stored in the memory.
[0061] If the step#1 cell search has been performed on all of the L
PDM hypotheses, M PDM hypotheses are determined on the basis of the
result energies of the step#1 cell search. A value determined by
averaging the determined M PDM hypotheses can be used as an initial
frequency offset estimation value. The PDM hypotheses are averaged
to reduce errors. A method for using a hypothesis having the
maximum energy as an initial frequency offset estimation value can
also be used.
[0062] When the step#1 cell search is performed on the L PDM
hypotheses, an order in which the PDM hypotheses are determined can
be implemented in several ways. For example, the step#1 cell search
can be performed in order of the smallest PDM hypothesis to the
largest PDM hypothesis, or from the largest PDM hypotheses to the
smallest PDM hypotheses. In addition, it is also possible to
randomly extract a plurality of hypotheses from the PDM hypotheses
and perform the step#1 cell search on the extracted hypotheses.
[0063] However, because a frequency offset driving a normal TCXO at
a room temperature commonly has the highest probability of `0`, it
is efficient that an initial PDM hypothesis is set to `0` and the
next hypotheses are set to befarther from `0`.
[0064] That is, preferably, an initial PDM hypothesis PH(0) is set
to 0 becoming PH(0)=0, a second hypothesis PH(1) is set to a point
increased by S from the PH(0) in a positive direction becoming
PH(1)=S, a third hypothesis PH(2) is set to a point decreased by S
in a negative direction becoming PH(2)=-S, in this manner,
PH(3)=2S, PH(4)=-2S, . . . .
[0065] Meanwhile, when the step#1 cell search is performed on each
of the PDM hypotheses, it is possible to obtain a more reliable
result by performing asynchronous accumulation (HN times) by
hardware and asynchronous accumulation (SN times) by software.
[0066] A description will now be provided of a method for
calculating a time required for the initial frequency offset
estimation according to an embodiment of the present invention. A
time required when the PEGC is operated one time is 2560+256=2826
chips. When asynchronous accumulation is performed HN times by the
hardware, HN*2826 chips are required, and when asynchronous
accumulation is performed SN times by software, SN*HN*2826 chips
are required. Because both perform the same procedure on L PDM
hypotheses, the total required time becomes L*SN*HN*2826 chips and
a software processing time is added thereto. For example, if a
9-bit PDM is used and a PDM space is S=20, then L=25 based on the
above formula, and if SN=2 and HN=10, a required time becomes
1413000 chips. The chip interval is converted into about 370 ms in
terms of time. In addition, if the software processing time is
added thereto, the chip interval becomes 400 ms. In actual
implementation, about 400-.about.420 ms is required.
[0067] With reference to FIG. 5, a description will now be made of
an initial frequency offset estimation method according to an
embodiment of the present invention.
[0068] FIG. 5 is a flowchart illustrating an initial frequency
offset estimation method using step#1 cell search according to an
embodiment of the present invention. Referring to FIG. 5, a
predetermined number (e.g., L) of PDM hypotheses are generated in
the method described in connection with FIG. 4 (Step 501). The
generated PDM hypotheses can be stored in the memory 211 of FIG. 2.
Thereafter, the step#1 cell search is performed on each PDM
hypothesis in the method described in connection with FIGS. 2 and 3
(Step 503). The step#1 cell search result on each of the PDM
hypotheses is also stored in the memory (Step 505). As previously
described, for efficient utilization of the memory, only a
predetermined number of PDM hypotheses having a maximum energy can
be stored in the memory.
[0069] When the step#1 cell search has been completed for all PDM
hypotheses, an initial frequency offset estimation value is
determined from the stored step#1 cell search result values, i.e.,
peak energies (Step 507). The initial frequency offset estimation
value can be determined as a PDM hypothesis having a maximum peak
energy, or determined by averaging a predetermined number of PDM
hypotheses having a higher peak energy.
[0070] A frequency offset is estimated based on the determined
initial frequency offset estimation value (Step 509), and the
process is performed by the controller 219 of FIG. 2 by selecting a
result value of the initial frequency offset estimator 213. After a
frequency offset is estimated by an initial frequency offset
estimation value, frequency offset estimation by an AFC becomes
possible. Therefore, frequency offset estimation is performed by
the AFC (Step 511). The frequency offset estimation by the AFC is
implemented by selecting an output value of the FDD 217 and
outputting the selected value to the accumulator 221 see FIG.
2.
[0071] Meanwhile, when frequency offset estimation is not normally
performed by the initial frequency offset estimation method
according to an embodiment of the present invention, it is
difficult to perform frequency offset estimation by the AFC. In
this case, it is also possible to repeatedly perform the initial
frequency offset estimation according to an embodiment of the
present invention.
[0072] As can be understood from the foregoing description,
embodiments of the present invention can rapidly acquire an
accurate estimation value in which an individual characteristic of
TCXO is reflected, in estimating an initial frequency offset for
initial cell search of a UE supporting a mobile communication
system. In addition, because an apparatus provided for step#1 cell
search can be used in performing an initial frequency offset
estimation according to an embodiment of the present invention,
extra hardware is not required.
[0073] While the invention has been shown and described with
reference to an embodiment thereof, it will be understood by those
skilled in the art that various changes in form and details may be
made therein without departing from the spirit and scope of the
invention as defined by the appended claims.
* * * * *